1. Field of the Invention
This invention relates to metering devices for discharging predetermined amounts of powder from a fluidized bed into containers.
2. Description of the Prior Art
Homogeneity and enrichment blending of UO.sub.2 powder is a major process step in the manufacturing of UO.sub.2 pellets. The fluidized bed blender produces almost ideal blends by incorporating the mixing action of gas bubbles to produce a homogeneous powder mixture. However, fluidization of a powder bed is characterized by the entire weight of the powder being suspended by the flow of gas rising through the bed. When fluidized powder is discharged from the bottom of such a fluidized bed blender, the gas and powder have an extremely high flow rate because of a gravitational pressure head created by the mass of fluidized material above the powder discharge outlet. A rapid flow rate of fluidizing gas from this outlet tends to destroy the homogeneous mixture by collapsing the fluidization state of the powder. Also, a rapid flow rate of material from the outlet makes it difficult to accurately meter fluidized material into containers.
It is generally necessary to limit the container weights of UO.sub.2 powder to a maximum of 35 kilograms to comply with nuclear safety and OSHA requirements. Testing has shown that the flow rates encountered in an unrestricted discharge from the bottom of a 500 kilogram fluidized bed can be as high as 5 kilograms per second through a typical 1.5 inch ball valve. It is difficult to meter 30 to 35 kilograms of powder into containers at this flow rate with any reasonable degree of accuracy, due to inertia forces of powder on the container and response time of valve closings.
In addition, it has been demonstrated that the flow of gas and powder through a ball valve during the discharge sequence produces pressure force fluctuations on the container, which are difficult to correct for when making container weight measurements. If these fluctuations are not compensated for, an apparent downward force is produced by the fluidizing gas. The downward force is a result of the gas pressure acting on the apparatus and container construction. A circular cross-section is removed from the middle of the container lid to serve as the inlet for the powder with the result that the surface area of the remainder of the lid is less than the surface area of the container bottom. Since the gas exerts an equal pressure on all areas of the container interior this differential in surface area results in an apparent downward force on the container. When the weight of the container and the powder contained therein is measured, this apparent downward force must be corrected for, or the measured weight of the container will be inaccurate.
The fluidizing action also causes the powder to become entrained in the fluidizing gas creating two additional problems. First, the powder which is suspended in the fluidizing gas must be separated and deposited in the bottom of the container. Second, the containers must exhaust a controlled amount of fluidizing gas and any residual powder in the form of dust suspended in the gas. This must be accomplished without significantly reducing the pressure in the container and without exhausting large amounts of powder and gas from which the powder must be extracted by filtering equipment to insure that the gas ultimately discharged into the atmosphere meets the very strict standards applied to the nuclear manufacturing establishments.
It is therefore an object of this invention to provide an improved powder discharge and metering method and apparatus for a fluidized bed blender which accurately meters the discharge of predetermined amounts of powder into containers while maintaining the homogeneity of the blended material.
Another object of this invention is to provide a fluidized bed powder discharge and metering method and apparatus which is substantially enclosed so as to lower the pressure drop across the discharge outlet and decrease the rate of gas outlet flow, so as to maintain the homogeneity of the blended material by maintaining fluidization.
Another object of this invention is to provide a fluidized bed powder discharge and metering method and apparatus which is substantially enclosed so as to lower the pressure drop across the discharge outlet and decrease the rate of gas outlet flow, permitting more accurate metering of the powder in discharge.
It is another object of this invention to correctly weigh the container, to minimize the effects of gas pressure fluctuation on the measured container weight and to terminate powder discharge when the container has been loaded with a predetermined weight of powder.
It is another object of this invention to provide a method and apparatus in which the powder is metered in short duration discharge periods, the powder deposited in the container is weighed during the intermittent, relatively quiescent periods between the powder discharges, and the discharge periods are terminated when the container has been loaded with a predetermined weight of powder.
It is another object of this invention to provide a fluidized bed powder discharge and metering apparatus which creates a substantially enclosed system during powder discharge and exhausts the excess fluidizing gas from the container while minimizing the amount of powder exhausted and released into the atmosphere.